1. Technical Field
The present invention relates to color image forming apparatuses which form color images on sheets by transferring toner images formed on photoreceptor drums on to a transfer member.
2. Description of Prior Art
Color image forming apparatuses such as copying machines, printers, facsimiles, etc., that use the electro-photography method, form images on sheets using, for example, a procedure such as the following. To begin with, an electrostatic latent image is formed by exposing a charged photoreceptor drum to light, and then a toner image is formed by making the electrostatic latent image visible by applying toner to the photoreceptor drum. Next, by transferring and fixing this toner image onto a sheet via a transfer member, a color image is formed on the sheet. As a means for exposure of the photoreceptor drum, apart from one that scans a laser beam along the main scanning direction, an LED print head that has a plurality of light emitting diodes arranged along the main scanning direction has been known conventionally.
A color image is formed, according to the above procedure, for example, by forming toner images of each color of yellow, magenta, cyan, and black, and by superimposing these one upon the other. If the position of superimposing each of the toner images gets shifted due to variations in the speed of rotation of the photoreceptor drum, a color shift (or color registration error) will occur in the color image formed on the sheet, and the image quality decreases.
In view of this, an image forming apparatus has been proposed that is provided with a rotation detection section which obtains the amount of movements in the rotation of the photoreceptor drum which is the image recording medium, and a recording timing control section that controls the recording timing for starting the recording of the image in a direction intersecting the rotation movement direction according to the amount of rotation movement obtained from the rotation detection section (see, for example, Unexamined Japanese Patent Application Publication No. Hei07-225544).
In paragraph 0036 of Unexamined Japanese Patent Application Publication No. Hei07-225544, it has been described that, even when any fluctuations are present in the rotational speed of the photoreceptor drum, it is possible to control the recording timing of starting the formation of the image in the main scanning direction of the photoreceptor drum so that the images transferred to the image transfer medium are at equal intervals in the direction of movement of rotation of the photoreceptor drum, and as a result of this, it is possible to eliminate density striations or image shifts.
Further, in Unexamined Japanese Patent Application Publication No. 2000-356875, an image forming apparatus has been proposed which controls the timing of starting the formation of images on the photoreceptor drum based on the timing of starting the conveying of transfer material.
Further, in Unexamined Japanese Patent Application Publication No. 2004-191600, an image forming apparatus has been proposed which corrects the shift in registration in the sub scanning direction that changes successively along with the rotation of the photoreceptor drum which is the image supporting member by converting the image data.
However, in any of the control methods of the above patent documents, the printing ratio of the color image formed on the sheet has not been taken into consideration for suppressing the color shift of the color image that is caused by the variation component of the photoreceptor drum. Therefore, since the angular speed of the photoreceptor drum varies according to the printing ratio of the color image, it is not possible to suppress the color shift in a direction perpendicular to the axis of the photoreceptor drum that is caused by the variation component of the photoreceptor drum in accordance with the printing ratio.